1. Field of the Invention
The present invention relates to an ultrasonic wave transmitter/receiver used as, for example, a clearance sensor and a back sensor for use in automobile.
2. Description of the Related Art
Hitherto, an ultrasonic wave transmitter/receiver is known in which a piezoelectric vibrating element is fixed on the inner bottom surface of a cylindrical case with a bottom, and which uses the outer bottom surface of the case as an ultrasonic wave transmitting/receiving surface. When using such an ultrasonic wave transmitter/receiver as a sensor for use in a car, if the transmission/reception range in the vertical direction is too wide with respect to the ground, reflected waves from the ground are detected as noises, and if the transmission/reception range in the horizontal direction is too narrow, the detection range has a blind spot. Neither of these is desirable. Accordingly, an ultrasonic wave transmitter/receiver has been invented which is arranged so that its transmission/reception range in the vertical direction is narrow and so that its transmission/reception range in the horizontal direction is wide, by forming the hollow portion of the case so as to be long in the vertical direction and short in the horizontal direction, and further by forming a thick-walled portion and thin-walled portions on the bottom of the case. In such an ultrasonic wave transmitter/receiver, for example, by forming a thick-walled portion with a thickness of 0.75 mm and thin-walled portions with a thickness of 0.3 mm on the case bottom with a diameter of 18 mm, and by disposing the thin-walled portions in the vertical direction, an ultrasonic wave transmitter/receiver wherein the transmission/reception range in the vertical direction is 40xc2x0, can be obtained.
FIG. 9 shows an example of an ultrasonic wave transmitter/receiver 1 used as a back sensor for use in a car. The transmission/reception range is expressed by a full angle at half maximum. The xe2x80x9cfull angle at half maximumxe2x80x9d refers to an angle formed by both directions at which a reduction of 20xc2x7log0.5 dB (about 6 dB) in the transmission/reception sensitivity occurs with respect to the transmission/reception sensitivity in the direction opposite to the bottom (the direction of 0xc2x0).
However, in such a conventional ultrasonic wave transmitter/receiver, since the reverberation characteristics are unstable and the reverberation time is long, ultrasonic waves reflected from an object adjacent to the ultrasonic wave transmitter/receiver can interfere with the reverberation thereof, so that there are cases where the existence of the adjacent object cannot be detected and/or where the distance to the adjacent body cannot be measured. In particular, when water drops are adhered on the transmitting/receiving surface, the reverberation time is prone to be longer, and hence, a problem occurs when using this ultrasonic wave transmitter/receiver as a sensor for use in a car, the sensor being exposed to wind and rain.
Accordingly, it is an object of the present invention to provide an ultrasonic wave transmitter/receiver in which the transmission/reception range thereof in at least one direction is relatively narrow, and which has stable reverberation characteristics.
In accordance with the present invention, there is provided an ultrasonic wave transmitter/receiver which comprises a cylindrical case with a bottom, a piezoelectric vibrator fixed on the inner bottom surface of the bottom of the case, and an ultrasonic wave transmitting/receiving surface defined by the outer bottom surface of the bottom of the case. In this ultrasonic wave transmitter/receiver, the bottom of the case includes a thick-walled portion on which the piezoelectric vibrator is fixed, and a thin-walled portion thinner than the thick-walled portion. When the thickness of the thick-walled portion is t1, the thickness of the thin-walled portion is t2, the thickness of a side portion provided continuously with the thin-walled portion is A, and the equations are set as x=t2/t1, and y=A/t2, this ultrasonic wave transmitter/receiver is formed so that x and y are within the range defined by the following equations:
0.50xe2x89xa6xxe2x89xa60.64
0.75xe2x89xa6yxe2x89xa61.75
yxe2x89xa6xe2x88x92(50/7)xc2x7x+163/28
yxe2x89xa7xe2x88x92(50/7)xc2x7x+135/28